The present invention relates to a fiber optic gyro in which clockwise and counterclockwise light beams are caused to propagate through an optical fiber coil and their phase difference is detected to thereby detect an angular rate applied to the optical fiber coil about the axis thereof.
FIG. 1 shows a conventional fiber optic gyro. Light from a light source 11 passes through an optical fiber coupler or similar optical coupler 12 and enters into a polarizer 13, wherein its component in a predetermined direction of polarization alone is extracted. The light from the polarizer 13 is split by an optical fiber coupler or similar optical coupler 14 into two, one of which is provided as a clockwise beam to one end of a single-mode optical fiber coil 16 via a depolarizer 15 and the other of which is provided as counterclockwise beam to the other end of the optical fiber coil 16 via an optical phase modulator 17. The clockwise and counterclockwise beams, after having propagated through the optical fiber coil 16, return to the optical coupler 14, wherein they are combined to interfere with each other. The resulting interference light is provided to the polarizer 13, wherein its component only in a predetermined direction of polarization is extracted. The light having thus passed through the polarizer 13 is branched by the optical coupler 12 into a photodetector 18 for conversion into an electric signal corresponding to the intensity of the interference light. A periodic function signal from a modulation signal generator 19, for example, a sine-wave signal, is applied to the optical phase modulator 17 to drive it, phase modulating the light passing therethrough. The output of the photodetector 18 is provided to a synchronous detector 21, wherein it is synchronously detected by a reference signal from the modulation signal generator 19, and the detected output is provided to an output terminal 22.
With no angular rate applied to the optical fiber coil 16 about its axis, no phase difference exists between the clockwise and counterclockwise light beams having propagated through the optical fiber coil 16 and the output of the synchronous detector 21 is also zero. When an angular rate is being applied to the optical fiber coil 16 about its axis, the phase difference corresponding to the angular rate is introduced between the clockwise and counterclockwise light beams and the synchronous detector 21 yields at output terminal 22 an output of a polarity and a level corresponding to the direction and magnitude of the applied angular rate, permitting detection of the applied angular rate.
In this way, the fiber optic gyro is to detect the phase difference between the clockwise and counterclockwise light beams. In the case where the depolarizer 15 is not provided, the state of polarization of the input linearly polarized light undergoes a change during propagation through the optical fiber coil 16, producing a component polarized perpendicularly thereto. Since the optical fiber coil 16 is a little birefringent due to its bend on itself, light beams of components polarized perpendicularly to each other propagate through the optical fiber coil 16 at different velocities. Consequently, when the one polarized component of the clockwise light beam and the other polarized component of the counterclockwise light beam, which are combined by the optical coupler 14, interfere with each other, the phase difference between the clockwise and counterclockwise light beams cannot be detected correctly.
To avoid this, it is a general practice in the prior art to provide the depolarizer 15, whereby the one polarized component and the other polarized component perpendicular thereto are made equal in intensity, displaced far apart in phase and rendered uncorrelative or incoherent to each other (that is, non-polarized relative to each other), thereby preventing that the one polarized component of the clockwise light beam and the other polarized component of the counterclockwise light beam interfere with each other.
The depolarizer 15 is usually a LYOT type fiber depolarizer (K. Bohm, et al., IEEE col. LT-1, No. 1, March 1983, p.71, for example). The depolarizer is produced by splicing two constant polarization optical fibers (plane-of-polarization retaining optical fibers, i.e. birefringent optical fibers), with major axes in their diametrical direction held at 45.degree. to each other.
There has also been proposed a fiber optic gyro of the type employing a polarization retaining optical fiber as the optical fiber coil 16, omitting the depolarizer 15. In this instance, the state of polarization of each of the clockwise and counterclockwise light beams, which propagate through the optical fiber coil 16, is held stable, and hence the gyro functions remain stable. The polarization retaining optical fiber has two low-melting-point tension cores disposed along the fiber core on both sides thereof in parallel, adjacent but spaced relation thereto. With a tensile stress applied to the fiber core in the direction of its diameter passing through the two tension cores, the optical fiber develops a birefringent property that its refractive index differs between the above-mentioned direction of the diameter of the fiber core and the direction of its diameter perpendicular thereto. The polarization retaining optical fiber is so expensive that the cost of the optical fiber coil 16 accounts for about 50% of the overall cost of the fiber optic gyro.
In the case of employing the single mode optical fiber as the optical fiber coil 16, the fiber optic gyro is less expensive but its fabrication is time- and labor-consuming because the depolarizer 15 is used in this case. That is, the depolarizer 15 is formed by fusion-splicing two polarization retaining optical fibers, with their principal axes displaced 45.degree. apart, and the accuracy required for the angular deviation between the principal axes is very high. Besides, a quadrapole winding, set forth in SPIE, Vol. 412, pp. 268-271 (1983), is effective in reducing a drift of the fiber optic gyro which is caused by a temperature change or similar external disturbance. With the quadrapole winding, however, it is necessary, for the reduction of the drift, that the influence of the external disturbance be held symmetrical with respect to the center of the optical fiber along the entire length thereof; hence, in the structure wherein the depolarizer is disposed at one end of the coil, the influence of the external disturbance is asymmetrical due to the presence of the depolarizer, resulting in insufficient reduction of the drift.